ALSA: hda: fix possible NULL dereference

After a for-loop was replaced by list_for_each_entry, see
Commit bbbc7e8 ("ALSA: hda - Allocate hda_pcm objects dynamically"),
Commit 751e221 ("ALSA: hda: fix possible null dereference"),
a possible NULL pointer dereference has been introduced; this patch adds
the NULL check on pcm->pcm, while leaving a potentially superfluous
check on pcm itself untouched.

Signed-off-by: Markus Osterhoff <linux-kernel@k-raum.org>
Cc: <stable@vger.kernel.org> #v4.1+
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/hda/hda_codec.c

@@ -3172,7 +3172,7 @@ static int add_std_chmaps(struct hda_codec *codec)
 			struct snd_pcm_chmap *chmap;
 			const struct snd_pcm_chmap_elem *elem;
 
-			if (!pcm || pcm->own_chmap ||
+			if (!pcm || !pcm->pcm || pcm->own_chmap ||
 			    !hinfo->substreams)
 				continue;
 			elem = hinfo->chmap ? hinfo->chmap : snd_pcm_std_chmaps;